1. Field of the Invention
The present invention relates to a mechanism for cooling a cast strand in continuous metal casting and, more particularly, to an air-water mist cooling method by which a cast strand can be uniformly cooled.
2. Description of the Prior Art
Although conventionally a water spraying method has been generally used for cooling a cast strand which is continuously pulled out in continuous metal casting, recently an air-water mist cooling method whereby the surface cracks of a cast strand are reduced and the quantity of the water to be consumed is relatively small and the cooling efficiency is high has become the main method utilized. Cooling of a continuous cast strand using an air-water mist spraying apparatus is performed in the manner such that an exhaust hole formed in an atomizing nozzle is disposed so as to be directed to the surface of the continuous cast strand from the portion between guide rollers, thereby spraying the mist toward the cast strand from the above portion between the guide rollers. In this case, it is desired to spray the mist over as wide a region as possible in order to uniformly cool the cast strand. However, using too large of a spreading angle .theta. of the spraying stream causes a part of the spraying mist to be cut off by the guide rollers, so that that part does not reach the cast strand and the mist would be ineffectively consumed, and at the same time there occurs a problem of overcooling of the guide rollers. Therefore, the spreading angle .theta. is controlled in accordance with the distance between the guide rollers, i.e., the angle .theta. is set at a value such that the outermost edges of the spraying mist almost coincide with the tangential directions of the guide rollers, so that the spreading angle .theta. of the spraying mist is extremely small. As a result, the surface of the cast strand to be directly cooled by the mist corresponds to only the narrow region to be covered by the small angle .theta., and the cooling efficiency of the regions before and behind this narrow region, is reduced since the cast strand in that region is indirectly cooled or is cooled by the air. Thus, the cooling rate of the cast strand, particularly the cooling rate of the surface portion of the cast strand instantaneously becomes nonuniform and variation occurs on the surface of the cast strand in the shrinkage portions due to the cooling, inviting an imbalance in stress, so that there is a problem that the cast strand frequently cracks (in particular, the surface thereof cracks). On the other hand, to be considered is a method disclosed in Japanese Patent Kokai (Laid-Open) No. 12347/82 whereby two exhaust holes open with a proper distance in an atomizing nozzle, thereby enlarging the mist spraying region as a whole. However, according to the experiments using such nozzle, it has been found that the cooling state of the cast strand was such that the quantity of the mist to be cut off by colliding with the guide rollers contrarily merely increased and, consequently, it was impossible to avoid the above-mentioned problem.
In addition, a cooling mechanism for use in continuous metal casting equipment comprises a header for supplying the cooling water (hereinbelow, referred to as a header), water branching pipes and atomizing nozzles and is disposed along a cast strand supporting apparatus such as guide rollers or the like. Since this cast strand supporting apparatus is vertically circular-arc-like shaped, there occurs a difference in water pressure at both upper and lower ends of the header. Therefore, in such supporting apparatus provided with the same atomizing nozzles, the resulting quantities of spraying water become uneven. More particularly, the water pressure difference when the quantity of the water is small provides a large influence. Thus, the cooling conditions in the longitudinal direction (pulling-out direction) of the cast strand become nonuniform and the cast strand cracks, causing the surface quality to deteriorate.
To solve such problems, as a method of making the quantities of the spraying water uniform, there has been conventionally known a method whereby a flow regulating valve as a fixed throttle or variable throttle is provided for each water branching pipe; the pressure loss is controlled by the opening angle of that valve; the bores of the headers are set at values which sequentially become smaller from the upper portion; pressure losses are caused in the headers; and the nozzle back pressures are reduced, thereby making the quantities of the water sprayed from all stages uniform.
However, according to the previously-mentioned conventional cooling apparatus, special tools or special processes are needed for the water branching pipes or headers. This causes the construction to be complicated and accidents may easily occur in the apparatus, thus requiring a significant amount of labor for the repair and inspection and at the same time resulting in a high production cost. Moreover, in case of an apparatus using throttles, there is a troublesome drawback in that the quantities of the spraying water have to be controlled to obtain the uniform quantities of the spraying water.